1. Field of the Invention
This invention relates to an imaging device suited for use in a camera capable of taking not only moving pictures but also still pictures, or taking moving pictures of different types.
2. Description of the Related Art
Some video cameras additionally have a function as an electronic still camera. Typical one of such video cameras includes a solid-state imaging element array having a matrix of 960 photosensor pixels in a horizontal direction (a row direction) and 640 photosensor pixels in a vertical direction (a column direction). Thus, the solid-state imaging element array corresponds to 640 lines. The typical video camera can selectively operate in a moving-picture taking mode or a still-picture taking mode. During the moving-picture taking mode of operation, the typical video camera generates an NTSC television signal from 525-NTSC-line signals obtained by the solid-state imaging element array for every frame. The typical video camera outputs the generated NTSC television signal. During the still-picture taking mode of operation, the typical video camera generates a still-picture signal from all-line signals (640-line signals) obtained by the solid-state imaging element array. The typical video camera outputs the generated still-picture signal. The resolution of a still picture taken by the typical video camera corresponds to about 600,000 (960 by 640) pixels.
To provide a higher picture resolution, it is known to increase the number of photosensor pixels composing a solid-state imaging element array to a range greater than 600,000. For example, a conceivable solid-state imaging element array has a matrix of 1280 photosensor pixels in a horizontal direction and 720 photosensor pixels in a vertical direction. The conceivable solid-state imaging element array corresponds to 720 lines. In the case where the 720 lines are progressively scanned for every frame with a frequency of 30 Hz, the conceivable solid-state imaging element array outputs a moving-picture signal of a 720P/30 frame system.
Generally, a solid-state imaging element array includes a CCD arrangement for transferring signal charges from photosensor pixels to an output terminal. The CCD arrangement has vertical signal-charge transfer portions (vertical CCDs) and a horizontal signal-charge transfer portion (a horizontal CCD). In the case where signal charges corresponding to 2560 (1280 multiplied by 2) pixels are transferred by a horizontal CCD for every horizontal scanning period of 63.5 μs, the horizontal transfer rate is equal to 40.31 MHz (=1280×2/63.5 μs). As the horizontal transfer rate in the horizontal CCD is increased, the rate of electric power consumed by the horizontal CCD rises and the rate of heat generated by the horizontal CCD also increases. An increase in the heat generation rate lowers the S/N ratio of a picture signal outputted from the solid-state imaging element array.
Japanese patent application publication number P2002-27332A discloses an imaging device having a solid-state imaging element array including photosensor pixels, vertical CCDs, and a horizontal CCD. Signal charges are transferred from the photosensor pixels to the horizontal CCD via the vertical CCDs before being outputted from the horizontal CCD. The imaging device in Japanese application P2002-27332A is designed to solve the above-mentioned problems, that is, an increase in the rate of electric power consumed by the horizontal CCD and a decrease in the S/N ratio of a picture signal outputted from the solid-state imaging element array. Specifically, the imaging device can selectively operate in a moving-picture taking mode or a still-picture taking mode. The solid-state imaging element array is divided into regions including regions B, D, and E. The regions B and E adjoin each other along a vertical line, and extend above the horizontal CCD. The region D is provided between the region E and the horizontal CCD. The region E is remoter from the output end of the horizontal CCD than the region B is. During the still-picture taking mode of operation, signal charges are transferred from the regions B, D, and E to the horizontal CCD. During the moving-picture taking mode of operation, signal charges are transferred from the region B to the horizontal CCD while the region D blocks the transfer of signal charges from the region E to the horizontal CCD. Thus, the number of pixels represented by signal charges transferred to the horizontal CCD during every horizontal scanning period of the moving-picture taking mode of operation can be reduced relative to that during the still-picture taking mode of operation. Accordingly, during the moving-picture taking mode of operation, the horizontal transfer rate in the horizontal CCD can be set at a relatively low value.
In the imaging device of Japanese application P2002-27332A, the region E in the solid-state imaging element array is effectively used during the still-picture taking mode of operation, and is not effectively used during the moving-picture taking mode of operation. Thus, the shape of an effective area (an actually used area) of the solid-state imaging element array and the position of the center thereof depend on whether the imaging device operates in the still-picture taking mode or the moving-picture taking mode. The imaging device includes a controller for moving the solid-state imaging element array relative to a lens to compensate for the change in the position of the center of the effective area of the solid-state imaging element array. Specifically, the controller shifts the position of the solid-state imaging element array relative to the lens depending on whether the imaging device operates in the still-picture taking mode or the moving-picture taking mode. Therefore, the position of the optical axis center of the lens relative to the solid-state imaging element array depends on whether the imaging device operates in the still-picture taking mode or the moving-picture taking mode.